Gluten-sensitive enteropathy is a genetically determined intolerance to dietary gluten, the harmful prolamins of wheat, rye and barley. The HLA alleles DQA1*0501 and DQB1*0201 encoding the HLA DQ2 heterodimer confer the genetic susceptibility to both celiac (also spelt coeliac) disease and dermatitis herpetiformis, the most commonly diagnosed disease forms belonging to this group. One or more so far unknown genes at HLA-unlinked loci most probably also predispose to celiac disease, where gluten ingestion leads to villous atrophy in the small bowel. The enteropathy is the result of gluten-triggered autoimmune processes and is self-perpetuating in the presence of gluten consumption. The immunological response is reduced and a mucosal healing is seen when gluten is excluded from the diet. The typical clinical manifestation of celiac disease is an overt malabsorption in young children or a severe wasting disease in adults. During the last decades it has become evident that celiac disease is underdiagnosed, the clinical features of the disease have changed to milder forms, and the diagnosis is often made at older age. Also in adult patients there is a shift towards milder symptoms. The true prevalence of celiac disease in different populations may be as high as one in 100.
The haplotype DR3-DQ2 is typical for many autoimmune disorders and is found in approximately 25% of the European population. Common celiac disease-associations are insulin-dependent diabetes mellitus, Sjögren's syndrome, autoimmune thyroiditis, rheumatoid arthritis, systemic lupus erythematosus and vitiligo. Further, the prevalence of autoimmune disorders in celiac disease is related to the duration of exposure to gluten. Untreated celiac disease patients also run an increased risk for malignancies and small-bowel lymphoma. Extraintestinal gluten-triggered manifestations are dermatitis herpetiformis, permanent-tooth enamel defects, osteopenia, liver involvement, cardiomyopathy, infertility, ataxia, and epilepsy with cerebral calcifications.
Small bowel biopsy is the cornerstone for celiac disease diagnosis. In addition to subtotal villous atrophy and crypt hyperplasia, a typical feature for the gluten-induced gut mucosal lesion is a high density of intraepithelial γδ+T lymphocytes. However, biopsy is an invasive diagnostic tool and is unsuitable for screening purposes. Certain serum autoantibodies, i.e. reticulin and endomysial antibodies, are gluten-induced, directed against the patient's own tissue extracellular matrix, and they are highly celiac disease-specific. Typically, these antibodies are found in IgA class, but celiac patients with selective IgA deficiency produce similar antibodies in IgG class (Collin P et al. Scand J Gastroenterol 1992;27:367-71). Recently, the enzyme tissue-type or cellular transglutaminase (tTG, EC 2.3.2.13, in the followings referred as transglutaminase) was identified as the target autoantigen of both endomysial and reticulin autoantibodies. Serological test based on the detection of these antibodies can be used to identify patients with mild or a typical symptoms and among subjects suffering of associated diseases, and even in the population. Case finding by screening has became the key in diagnosing gluten-enteropathy in recent years and some countries have started even mass screening of the school age population. There is an accumulating evidence, that the spectrum of gluten-sensitive diseases is broader than the classical gluten-enteropathy and may represent a general health care issue.
The reliable and reproducible perfomance of these antibody tests has only been achieved in specialized laboratories with the current methods. The reticulin and endomysial antibody assays require frozen tissue substrates, immunfluorescent equipments and a highly trained personnel to visually evaluate the antibody binding results. Importantly, the transglutaminase-based ELISA tests yield observer non dependent and quantitative results. However, their crucial component is the transglutaminase antigen which should preferably be as similar to the natural human autoantigen as possible. The Ca2+-induced catalytically active conformation seems to be the preferred antigen recognized by the majority of patient autoantibodies, but the epitope specificity of individual patients may differ. It is difficult to maintain the correct folding of the enzyme during purification from animal tissues or to achieve it when producing recombinant transglutaminase proteins. In addition, transglutaminase is very sensitive to storage even at −40° C. and therefore the continuos need for appropriate and fresh antigen lots may make the transglutaminase antibody test very expensive. Commercially only tissue transglutaminase purified from guinea pig liver (Sigma) is available, but this rodent transglutaminase was found to be less sensitive in detecting celiac antibodies than the natural human enzyme prepared from human red blood cells (Hansson et al. J Pediatr Gastroenterol Nutr 2000;30:379-84).
There is a need for a simple serological screening test for celiac disease and related gluten-sensitive disease entities. Such a test would preferably be performed when the first suspicion for the celiac condition or other gluten-sensitive disease entities arises, thus in the primary case or as a mass-screening. It should also be applicable to following-up of already diagnosed patients. The present invention now provides a quick, cheap and accurate method of diagnosing gluten-induced diseases.